The present invention involves an improvement to the feed and product in a benzene saturation process. In particular the present invention provides an adsorbent that is effective for trace sulfur removal for feeds that comprise a mixture of phases to benzene saturation units as well as product streams to such units.
For most refiners, the issue of benzene in the gasoline pool is one of managing benzene production from the catalytic reformer. The two primary strategies to accomplish this goal include the minimization of benzene and benzene precursors in the catalytic reformer feed, or the elimination of the benzene from the reformate after it is formed. A benzene saturation unit can be applied in either of these strategies. For example, a benzene saturation unit can be located on the overhead stream of a naphtha splitter, to remove the natural benzene concentrated by aggressive reformer feed prefractionation. Alternatively, a benzene saturation unit can be used on a light reformate stream to remove the benzene that has been produced in the reformer.
The benzene saturation process was developed as a low-cost, stand-alone option to treat C5-C6 feedstocks that are high in benzene. Benzene is saturated with hydrogen to make C6 naphthenes. The catalyst used in this process is highly selective for benzene saturation to C6 naphthenes. Makeup hydrogen is provided in an amount slightly above the stoichiometric level required for benzene saturation. The heat of reaction associated with benzene saturation is carefully managed to control the temperature rise across the reactor. Use of a relatively high space velocity in the reactor contributes to the unit's cost-effectiveness. A benzene saturation unit can be located on a light reformate or light straight-run naphtha stream.
The operation of a benzene saturation unit is very sensitive to the presence of sulfur-containing compounds. The specification and catalyst sensitivity requires extremely low levels of sulfur with preferably less than 50 parts per billion weight of sulfur. It has been found that copper oxides are more effective than other oxides such as zinc oxide and nickel oxide in removing sulfur. Prior art copper oxides had the disadvantage of being reduced to copper metal during operation. This not only decreased their effectiveness in removing sulfur compounds, but since the reduction process is highly exothermic, when used in connection with gases that have a low specific heat, the temperature exotherm can result in unsafe conditions, especially on start-up.
In a prior art design for a benzene saturation unit there has been a separate sulfur guard bed on the naphtha feed, and if warranted by the sulfur content, a second one on the makeup hydrogen stream as well. Separate guard beds have been required due to previous sulfur adsorbents needing to operate in single phase, either vapor or liquid, to achieve the required outlet sulfur level. If a sulfur adsorbent could achieve the required degree of sulfur removal in a mixed phase stream comprising vapor and liquid, it would be possible to instead have a compound bed, with the sulfur adsorbent on top, and the Pt on alumina benzene saturation catalyst below it. This would save capital cost as no separate sulfur guard beds or the related heat exchangers would be needed.
Guard beds with supported copper oxide (CuO) are often used for feed purification in benzene saturation units. Unfortunately, the CuO reduces to a lower valence state, at the typical operating temperatures in the range of ambient temperature for a make-up hydrogen treater and 120° to 150° C. for liquids being treated. Typically in prior art systems, the reduction of CuO occurs rapidly, and large amounts of water are produced. The excessive moisture is disadvantageous to the operation of the benzene saturation catalyst. In addition, there is the undesired exotherm.
Copper containing materials are widely used in industry as catalysts and sorbents. The water shift reaction in which carbon monoxide is reacted in presence of steam to make carbon dioxide and hydrogen as well as the synthesis of methanol and higher alcohols are among the most practiced catalytic processes nowadays. Both processes employ copper oxide based mixed oxide catalysts.
Copper-containing sorbents play a major role in the removal of contaminants, such as sulfur compounds and metal hydrides, from gas and liquid streams. One new use for such sorbents involve the on-board reforming of gasoline to produce hydrogen for polymer electrolyte fuel cells (PEFC). The hydrogen feed to a PEFC must be purified to less than 50 parts per billion parts volume of hydrogen sulfide due to the deleterious effects to the fuel cell of exposure to sulfur compounds.
Copper oxide (CuO) normally is subject to reduction reactions upon being heated but it also can be reduced even at ambient temperatures in ultraviolet light or in the presence of photochemically generated atomic hydrogen.
The use of CuO on a support that can be reduced at relatively low temperatures is considered to be an asset for some applications where it is important to preserve high dispersion of the copper metal. According to U.S. Pat. No. 4,863,894, highly dispersed copper metal particles are produced when co-precipitated copper-zinc-aluminum basic carbonates are reduced with molecular hydrogen without preliminary heating of the carbonates to temperatures above 200° C. to produce the mixed oxides. However, easily reducible CuO is disadvantageous in some important applications, such as the removal of hydrogen sulfide from gas and liquid streams when very low residual concentration of H2S in the product is required
The residual H2S concentration in the product gas is much higher by the laws of thermodynamics (which is undesirable) when the CuO reduces to Cu metal. in the course of the process since reaction (1) is less favored than the CuO sulfidation to CuS.2Cu+H2S═Cu2S+H2  (1)The known approaches to reduce the reducibility of the supported CuO materials are based on combinations with other metal oxides such as Cr2O3. The disadvantages of the approach of using several metal oxides are that it complicates the manufacturing of the sorbent because of the need of additional components, production steps and high temperature to prepare the mixed oxides phase. As a result, the surface area and dispersion of the active component strongly diminish, which leads to performance loss. Moreover, the admixed oxides are more expensive than the basic CuO component which leads to an increase in the sorbent's overall production cost.
The present invention comprises a new method to improve feed purification in a benzene saturation process by using a supported CuO adsorbent which contains chloride as a means to decrease the tendency of CuO to be reduced to low valent state, especially Cu metal. Surprisingly, it has now been found that introducing chloride either in the basic copper carbonate, which serves as CuO precursor, or into the intermediate CuO-alumina adsorbent leads to material having improved resistance to reduction by under high pressure hydrogen. This feature is especially useful in the benzene saturation process. In addition, it has been found that the adsorbent used in the present invention can remove sulfur impurities from mixed vapor/liquid operation to the levels necessary to protect the benzene saturation catalyst.